Comparison of coastal and river releases of Atlantic salmon smolts in the river Kokemaenjoki, Baltic Sea

Paired releases of Carlin tagged fish were carried out in the river Kokemae njoki, Bothnian Sea, in 1991-1993, to compare the survival and migration of Atlantic salmon (Salmo salar L.) smelts released at two different sites, o ff the estuary and at Nakkila, 55 km upstream. The eight different smelt gr oups, divided between the two sites, ranged in mean weight from 27 g (1 yr) to 260 g (2 yr). The size of the smelt groups were planned according to pr evious experience of the expected rate of recapture to allow the detection of a deviation of 20-25% from the initial 50:50 (coast:river) stocking rati o at a 5% significance level and 90% power, which requires 279 and 180 reca ptures for each group, respectively. A 25% deviation corresponds to 40% mor tality/55 km of river migration, which is somewhat less than reported in th e literature. In all, 20000 smelts were released. Tag recoveries were collected until the end of 1996 (total number 2544). Fo r all six groups of 2 yr smelts (120-260 g), the numbers recaptured were as high or higher than expected, exceeding the limit of 279 recaptures. For t he two groups of 1 yr smelts (27 and 34 g), the rates of recapture were low er than expected (only 20 and 14 recaptures, respectively), and therefore t he planned statistical analysis was not possible for these groups. For all paired releases of 2 yr fish, the results were similar irrespective of the release site. First, the distribution of recoveries over the years was homogenous for coastal and river releases at the 5% significance level (G-test for independence), suggesting that it is appropriate to add up all recoveries over time. Secondly, the total amounts of recoveries did not dif fer significantly from the original stocking ratios (coast:river) at the 5% significance level (G-test for goodness of fit), indicating that the relea se site did not affect the survival of 2 yr smelts. Due to extremely intensive sea fishery, 99.5% of adult recoveries with a kn own location (total 1695) were reported from the sea area. Only eight fish (0.5%) were captured in the Kokemaenjoki, and none in other rivers. It was thus not possible to test properly for differences in homing and straying b etween the two release sites. The migration pattern in the sea area was not affected by the release site. To improve the survival rates and the cost effectiveness of sea ranching, s almon smelts are released directly into the sea in the vicinity of several Baltic rivers. The present study challenges the economic justification of s uch releases in the River Kokemaenjoki. For large 2 yr smelts, comparable s urvival rates in this river system may be attained in upstream releases as well. (C) 1998 International Council for the Exploration of the Sea.